Multi-point opportunistic beamforming with selective beam attenuation

ABSTRACT

A method for communication includes receiving at a receiver from a group of two or more transmitters multiple Radio Frequency (RF) transmission beams that alternate in time and space and include at least first and second transmission beams. The method identifies that the first transmission beam causes interference to reception of the second transmission beam. Feedback is sent from the receiver to one or more of the transmitters, so as to cause the transmitters to attenuate the first transmission beam during transmission of the second transmission beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/723,647, filed Mar. 14, 2010, which claims the benefit of U.S.Provisional Patent Application 61/171,328, filed Apr. 21, 2009. Thedisclosures of these related applications are incorporated herein byreference.

FIELD OF THE DISCLOSURE

The present invention relates generally to communication systems, andparticularly to methods and systems for communication using multipleantennas.

BACKGROUND

Some communication systems transmit data from a transmitter to areceiver over multiple communication channels, using multiple transmitantennas and multiple receive antennas. Multiple-channel transmission isused, for example, in spatial multiplexing schemes that achieve highthroughput, in beam-forming schemes that achieve high antennadirectivity and in spatial diversity schemes that achieve highresilience against channel fading and multipath. These schemes are oftenreferred to collectively as Multiple-Input Multiple-Output (MIMO)schemes.

MIMO schemes are contemplated, for example, for use in Evolved UniversalTerrestrial Radio Access (E-UTRA) systems, also referred to as Long TermEvolution (LTE) systems. The Third Generation Partnership Project (3GPP)E-UTRA standards specify MIMO schemes for use by E-UTRA User Equipment(UE) and base stations (eNodeB's). These schemes are described, forexample, in 3GPP Technical Specification 36.211, entitled “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation (Release 8),”(3GPP TS 36.211), version 8.6.0, March, 2009, which is incorporatedherein by reference. The 3GPP is currently in the process of specifyingan extension of the E-UTRA specification, which is referred to asLTE-Advanced (LTE-A).

Opportunistic beam-forming is a communication technique in which atransmitter transmits a pattern of directional transmission beams thatalternates over time. The transmitter determines a suitable schedulingfor transmitting to a particular receiver, for example the optimalscheduling, based on feedback from the receiver. Opportunisticbeam-forming schemes are described, for example, by Viswanath et al., in“Opportunistic Beamforming Using Dumb Antennas,” IEEE Transactions onInformation Theory, volume 48, No. 6, June, 2002, pages 1277-1294, andby Sharif and Hassibi, in “On the Capacity of MIMO Broadcast Channelswith Partial Side Information,” IEEE Transactions on Information Theory,volume 51, No. 2, February, 2005, pages 506-522, which are incorporatedherein by reference.

The description above is presented as a general overview of related artin this field and should not be construed as an admission that any ofthe information it contains constitutes prior art against the presentpatent application.

SUMMARY

An embodiment that is described herein provides a method forcommunication. The method includes receiving at a receiver from a groupof two or more transmitters multiple Radio Frequency (RF) transmissionbeams that alternate in time and space and include at least first andsecond transmission beams. The method identifies that the firsttransmission beam causes interference to reception of the secondtransmission beam. Feedback is sent from the receiver to one or more ofthe transmitters, so as to cause the transmitters to attenuate the firsttransmission beam during transmission of the second transmission beam.

In some embodiments, the first and second transmission beams alternatein time and space in accordance with a pattern that is coordinated amongthe transmitters. In an embodiment, the first transmission beam isidentified as causing the interference after identifying the secondtransmission beam as being preferable for receiving subsequenttransmissions to the receiver. In a disclosed embodiment, the methodincludes identifying the second transmission beam as being preferablefor receiving subsequent transmissions to the receiver, and sending thefeedback includes sending a request to receive the subsequenttransmissions over the second transmission beam.

In some embodiments, identifying that the first transmission beam causesthe interference includes measuring signal quality on at least some ofthe received transmission beams, and detecting the interferenceresponsively to the measured signal quality. In an embodiment, measuringthe signal quality includes receiving pilot signals on the at least somereceived transmission beams, and measuring the signal quality over thepilot signals.

In an embodiment, identifying that the first transmission beam causesthe interference includes predicting the interference for a futureoccurrence of the first and second transmission beams, and sending thefeedback includes causing the transmitters to attenuate the firsttransmission beam during the future occurrence. In a disclosedembodiment, the method includes selecting a preferred time interval forreceiving the signals on the second transmission beam, and sending thefeedback includes causing the transmitters to attenuate the firsttransmission beam during the preferred time interval.

In an embodiment, receiving the transmission beams includes receivingsignals conforming to a Long Term Evolution (LTE) specification. Inanother embodiment, receiving the transmission beams includes receivingat least one transmission beam that is transmitted jointly by two ormore of the transmitters. In yet another embodiment, sending thefeedback includes causing the transmitters to transmit data to at leastone other receiver over the attenuated first transmission beam.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a communication apparatus including a receiver and aprocessor. The receiver is configured to receive from a group of two ormore transmitters multiple Radio Frequency (RF) transmission beams thatalternate in time and space and include at least first and secondtransmission beams. The processor is configured to identify that thefirst transmission beam causes interference to reception of the secondtransmission beam, and to cause sending of feedback to one or more ofthe transmitters, so as to cause the transmitters to attenuate the firsttransmission beam during transmission of the second transmission beam.In an embodiment, a mobile communication terminal includes the disclosedcommunication apparatus. In an embodiment, a chipset for processingsignals in a mobile communication terminal includes the disclosedcommunication apparatus.

There is also provided, in accordance with an embodiment that isdescribed herein, method for communication. The method includestransmitting to a receiver from two or more transmitters multipletransmission beams that alternate in time and space and include at leastfirst and second transmission beams. Feedback from the receiver isreceived at the transmitters. The feedback indicates that the firsttransmission beam causes interference to reception of the secondtransmission beam. Responsively to the feedback, the first transmissionbeam is attenuated during transmission of the second transmission beam.

There is further provided, in accordance with an embodiment that isdescribed herein, a communication system including two or moretransmitters. The transmitters are configured to transmit to a receivermultiple transmission beams that alternate in time and space and includeat least first and second transmission beams, to receive from thereceiver feedback indicating that the first transmission beam causesinterference to reception of the second transmission beam, and,responsively to the feedback, to attenuate the first transmission beamduring transmission of the second transmission beam.

The present disclosure will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a communicationsystem that employs opportunistic beam-forming with selective beamattenuation, in accordance with an embodiment of the present disclosure;

FIG. 2 is a flow chart that schematically illustrates a method forcommunication using opportunistic beam-forming with selective beamattenuation, in accordance with an embodiment of the present disclosure;and

FIGS. 3 and 4 are graphs showing transmission protocols that usecoordinated patterns of transmission beams, in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments that are described hereinbelow provide improved methods andsystems for opportunistic beam-forming. In some embodiments, acommunication system comprises two or more transmitters that transmit toa plurality of receivers. The transmitters transmit multiple directionalRadio frequency (RF) transmission beams, which alternate in time andspace. In some embodiments, the transmission beams alternate inaccordance with a pattern that is coordinated among the transmitters.

In some embodiments, each receiver receives the transmission beams andmeasures signal quality on the received transmission beams. Based on thesignal quality measurements, the receiver identifies one or morepreferred transmission beams over which it prefers to receive data, andone or more interfering beams that cause interference to the preferredbeams. Typically although not necessarily, the receiver first identifiesa preferred beam, and then identifies one or more beams that causeinterference to the preferred beam.

The receiver transmits feedback to the transmitters, which indicates thepreferred and interfering beams. In an embodiment, the receivertransmits a request to attenuate the one or more interfering beams.Alternatively, the receiver reports the signal quality measurements asfeedback, so as to enable the transmitters to select over which beam totransmit and which beam to attenuate.

In some embodiments, the transmitters take various actions and applyvarious policies in response to the feedback received from thereceivers. In an embodiment, the transmitters select over which beam orbeams to transmit to a given receiver. As another example, based onreceived feedback, the transmitters decide to attenuate one or more ofthe interfering beams while transmitting one or more of the preferredbeams. As yet another example, the transmitters select a preferred timeat which to transmit to a given receiver over a certain preferred beam.Various other transmitter decisions and decision criteria are describedherein.

Unlike some opportunistic beam-forming schemes, in accordance with anembodiment of the disclosure, the transmitters proactively attenuatetransmission beams that cause interference based on feedback relating toone or more received signals and/or specific requests from the receiverto attenuate one or more of the downlink signals. As a result, thedisclosed techniques provide high signal quality, high throughput andsmall latency. The disclosed techniques are suitable for communicationsystems whose performance is limited by interference, such as somecellular networks. Performance gains achieved by embodiments of thepresent disclosure are achieved with a relatively small increase incommunication overhead and system complexity.

FIG. 1 is a block diagram that schematically illustrates a wirelesscommunication system 20 that employs opportunistic beam-forming withselective beam attenuation, in accordance with an embodiment of thepresent disclosure. System 20 operates in accordance with any suitablecommunication standard or protocol. In the present example, system 20operates in accordance with the LTE or LTE-A specifications, citedabove. Alternatively, the techniques described herein can be used insystems operating in accordance with any other suitable communicationstandard or protocol, such as IEEE 802.16 (also referred to as WiMAX),Wideband Code Division Multiplexing (WCDMA) and Global System for Mobilecommunications (GSM), for example.

System 20 comprises multiple transmitters, which transmit signals tomultiple receivers. In the present example, the transmitters areembodied in LTE or LTE-A base stations (eNodeB's) 24, and the receiverscomprise LTE or LTE-A compliant mobile terminals (UE's) 28. AlthoughFIG. 1 shows three eNodeB's and a single UE for the sake of clarity,real-life systems typically comprise a large number of eNodeB's andUE's. Although the embodiments are described herein in the context ofdownlink transmissions from the eNodeB's to the UE's, in somecommunication systems the disclosed techniques are suitably adapted tothe uplink.

Each eNodeB 24 comprises a downlink transmitter 32, which transmitsdownlink signals to UE's 28 via one or more antennas 36. (In FIG. 1, theinternal eNodeB structure is shown in detail for only one of theeNodeB's, for the sake of clarity. The other eNodeB's in the systemtypically have a similar structure.) At any given time, the eNodeB's 24in system 20 transmit M downlink Radio Frequency (RF) transmissionbeams, which convey respective downlink signals that carry data to UE's28. The transmission beams (referred to herein as beams for brevity) aretypically directional.

In an embodiment, a directional transmission beam is produced by asingle eNodeB using two or more antennas 36. Alternatively, adirectional beam is produced by multiple eNodeB's that transmit the samesignal simultaneously, in which case each eNodeB uses one or moreantennas 36. The group of eNodeB's producing a given beam is referred toherein as a cluster. In the description that follows, each beam isregarded as being transmitted by a cluster of eNodeB's. When a beam istransmitted by a single eNodeB, the cluster is regarded as containingonly the single eNodeB.

Each transmission beam thus comprises a directional RF signal thatcarries data. Each beam is generated by transmitting the samedata-carrying signal from a set of antennas 36, which belong to one ormore eNodeB's, while applying respective multiplicative weights to theantennas in the set. In an embodiment, the direction in which a beam istransmitted is modified by modifying the weights applied to theantennas. This action is referred to as beam steering.

The beam steering applied to the i^(th) beam (i.e., the weights appliedto the different antennas that participate in transmission of the i^(th)beam) is represented by a weight vector denoted w_(i). If the cluster ofeNodeB's that generate the i^(th) beam is denoted B_(i), and the numberof antennas in the k^(th) eNodeB in this cluster is denoted N_(k), thenvector w_(i) has Σ_(kεB) _(i) N_(k) elements. Each element of w_(i) is amultiplicative weight (typically a complex number having a magnitude andphase) that is applied to the signal transmitted via the respectiveantenna 36. In some embodiments, the weight vectors in a given clusterare orthogonal to one another, i.e., w_(i) ^(H)w_(j)=0 if B_(i)=B_(j)(i≠j).

The set of M beams, which are transmitted by system 20 at a given time,is referred to herein as a beam setting. In some embodiments, system 20modifies the beam setting at periodic time intervals. In other words,eNodeB's 24 transmit multiple transmission beams, which alternate overtime and space. In LTE or LTE-A systems, for example, the beam settingcan be modified every Transmission Time Interval (TTI), every severalTTIs, or at any other suitable period.

In some embodiments, the eNodeB's alternate the beams in time and spaceaccording to a pattern that is coordinated among the eNodeB's. Inalternative embodiments, each eNodeB alternates the beams withoutcoordination with other eNodeBs. Further alternatively, the eNodeB's maynot use any sort of pattern. Typically, however, the eNodeB'ssynchronize the OFDM symbol timing with one another.

As will be explained below, in some embodiments the pattern of beamsettings is predefined, whereas in other embodiments the pattern ispseudo-random. When using a periodic pattern, each eNodeB may change itslocal beam settings (i.e., the weight vector elements corresponding toits own antennas 36) in a certain cycle period, which may be the same asor different from the cycle periods of other eNodeB's. Typically, thebeam settings are chosen to be sufficiently diverse, so that each UE islikely to encounter beam settings that provide high signal strength andlow interference. In some embodiments, a cluster of eNodeB's generatesand alternates the beams using an optimization process. The optimizationprocess is often unknown to the UEs.

When preparing to transmit data to a given UE 28, in an embodimentsystem 20 selects over which beam and at what time to transmit the data.Moreover, in some embodiments system 20 decides to attenuate one or moreof the beams so as to reduce interference to another beam. In accordancewith an embodiment of the disclosure, these scheduling and attenuationdecisions are based on feedback that is provided by the UE's, usingmethods that are described in detail herein.

Each eNodeB 24 comprises an uplink receiver 40, which receives uplinksignals from the UE's (either via antennas 36 or via separate receiveantennas, not shown in the figure). In particular, receiver 40 receivesfrom the UE's feedback, which enables the eNodeB's to make schedulingand beam attenuation decisions. In some embodiments, the feedbackcomprises an explicit request from the UE to receive transmissions overone or more beams and/or to attenuate one or more beams. Alternatively,the feedback comprises signal quality measures, as measured by the UE onone or more of the received beams. Using this information, the eNodeB'sdecide over which beams to transmit and which beams to attenuate.

Each eNodeB 24 further comprises a controller 44, which manages theoperation of the eNodeB. Controller 44 comprises a beam patterngenerator 48, which generates the pattern of beam settings to betransmitted from transmitter 32. In an embodiment, the beam patterngenerators 48 of the different eNodeB's 24 coordinate and synchronizethe pattern generation with one another.

In some embodiments, controller 44 comprises a beam attenuation module52, which configures transmitter 32 to attenuate one or more of thetransmitted beams. The beams to be attenuated are selected based on thefeedback received from UE's 28. Controller 44 further comprises ascheduler 56, which schedules data for transmission over the differentbeams. In particular, scheduler 56 decides over which beam to transmitdata to each UE, and at what time (e.g., during which TTI). In someembodiments, controller 44 (e.g., using scheduler 56) selects theappropriate modulation and error correction coding scheme to be used fortransmitting to each UE.

Each UE 28 receives the downlink transmission beams using one or moreantennas 60. A downlink receiver 64 receives and decodes the signalstransmitted over the different beams, so as to reconstruct and outputthe downlink data. In addition, receiver 64 measures the signal qualityon each beam. In an embodiment, receiver 64 measures, for example, theSignal to Noise Ratio (SNR) on each beam, or any other suitable signalquality measure.

As seen in FIG. 1, in an embodiment UE 28 comprises a processor 72,which manages and controls the operation of the UE. In some embodiments,processor 72 comprises a feedback calculation module 76, whichdetermines the feedback to be transmitted from the UE to the eNodeB's.In an example embodiment, Module 76 determines the feedback based on thesignal quality measurements performed by downlink receiver 64 on thedifferent beams. In some embodiments, the feedback indicates (1) one ormore beams that the UE regard as preferable for receiving downlinktransmissions, (2) one or more beams that cause interference to downlinkreception at the UE, and/or (3) a specific request to attenuate one ormore of the transmitted beams.

This sort of feedback enables the eNodeB's to schedule downlink dataover the preferred beams, and to attenuate interfering beams. Severalexamples of selection criteria and feedback schemes are describedfurther below. UE 28 comprises an uplink transmitter 68, which transmitsuplink signals to the eNodeB's. In particular, the uplink transmittertransmits the feedback produced by module 76.

The system configuration shown in FIG. 1 is a simplified exampleconfiguration, which is depicted for the sake of conceptual clarity. Inalternative embodiments, any other suitable system configuration canalso be used. In an embodiment, the different components of eNodeB's 24and UE's 28 are implemented using dedicated hardware, such as using oneor more Application-Specific Integrated Circuits (ASICs) and/orField-Programmable Gate Arrays (FPGAs). Alternatively, in an embodiment,some eNodeB and UE components are implemented using software running ongeneral-purpose hardware, firmware, or using a combination of hardwareand software elements.

In some embodiments, controller 44 and processor 72 comprisegeneral-purpose processors, which are programmed in software to carryout computer instructions to provide the functions described herein,although they too may be implemented on dedicated hardware. The softwareinstructions may be downloaded to the processors in electronic form,over a network, for example. Alternatively or additionally, the softwareinstructions are provided and/or stored on tangible media, such asmagnetic, optical, or electronic memory. In some embodiments, some orall of the elements of UE 28, and/or some or all of the elements ofeNodeB 24, are fabricated in a chip-set. UE and eNodeB elements that arenot mandatory for explanation of the disclosed techniques, such asvarious Radio Frequency (RF) elements, have been omitted from FIG. 1 forthe sake of clarity.

FIG. 2 is a flow chart that schematically illustrates a method forcommunication using opportunistic beam-forming with selective beamattenuation, in accordance with an embodiment of the present disclosure.The method begins at a beam transmission operation 80, with eNodeB's 24transmitting a pattern of RF transmission beams. At a beam receptionoperation 84, UE 28 receives at least some of the beams using downlinkreceiver 64. At a quality reception operation 88, receiver 64 measuresthe signal quality on each of the received beams. Receiver 64 reportsthe signal quality measures of the different beams to processor 72.

At a beam identification operation 92, processor 72 identifies one ormore preferable beams and/or one or more interfering beams, based on thesignal quality measurements. At a feedback operation 96, uplinktransmitter 68 transmits the feedback indicative of the preferred andinterfering beams to the eNodeB's. At a scheduling and attenuationoperation 100, eNodeB's 24 schedule subsequent transmissions to the UEbased on the received feedback. In addition, the eNodeB's select andattenuate one or more of the beams based on the UE feedback. Note thatthe eNodeB's in a given cluster typically cooperate in making schedulingdecisions, regardless of whether they coordinate the pattern of beamsettings with one another.

UE's 28 suitably calculate and report the feedback in various ways. Forexample, in an embodiment the UE calculates the SNR on each beam,assuming all the other beams are active (and therefore potentially causeinterference). In addition, in an embodiment the UE calculates the SNRon each beam, assuming that one or more of the worst-interfering beamsare turned off. In some embodiments, each UE is pre-allocated a set ofbeams, referred to as the serving beams of the UE. The serving beams ofa given UE may be generated by a single eNodeB, by a cluster ofeNodeB's, or by multiple clusters. In an embodiment, the UE measuressignal quality and calculates the feedback while considering only itspre-allocated serving beams (i.e., disregards potential interferencefrom beams other than the serving beams).

In an example embodiment, the UE indicates the most desirable beams inthe feedback by reporting the m strongest beams, m≧1. In someembodiments, the UE reports the SNR measured on these beams, as well. Insome embodiments, the UE identifies v≧1 preferable transmission timesover the preferable beams. In an embodiment, The UE reports thepreferable transmission times in the feedback, possibly together withthe achievable SNR at these transmission times.

In some embodiments, the UE identifies one or more beams that causeconsiderable interference to the reception of the preferable beams. Forexample, in an embodiment the UE identifies one or more beams, whoseattenuation (e.g., silencing) would cause the highest improvement in SNRover the preferable beams. In an example embodiment, the UE reports theu≧1 beams that cause the strongest interference. In another exampleembodiment UE also reports the SNR improvement that is expected toresult from attenuating the interfering beams. As another example, theUE reports all interfering beams, whose attenuation would improve theSNR over some preferred beam by at least Δ dB.

In some embodiments, a group of two or more beams is addressed to agiven UE, and the UE comprises a mechanism for canceling theinterference between different beams in the group (e.g., by decoding thesignals of the different beams and subtracting one signal from another).In these embodiments, the UE disregards interference between beamswithin the group when identifying the preferable and interfering beams,assuming it will be able to cancel this interference internally.Additionally or alternatively, the UE may identify and report thepreferable and interfering beams in any other suitable way.

Receiver 64 in the UE may measure the signal quality on the differentbeams in any suitable way. For example, in an embodiment the downlinksignal transmitted over a given beam comprises pilot symbols that areuniquely associated with that beam. Receiver 64 measures the signalquality on a given beam by estimating the achievable SNR using the pilotsymbols of the beam. For example, the receiver may estimate the channelresponse between the eNodeB's and the UE using the pilot signals. Basedon the estimated channel and the known beam steering vector (weightvector), the UE estimates the achievable SNR on that beam.

The eNodeB's 24 of system 20 receive the above-described feedback fromthe different UE's 28, and configure the transmission beams in variousways based on the feedback. The eNodeB's suitably decide, for example,over which beam or beams to transmit to each UE and at which times,which interfering beams to attenuate and at which times, the actualattenuation level to applied to each attenuated beam, which modulationand coding scheme to use in transmitting to a given UE, and/or any otherrelevant decision. When making these decisions, the eNodeB's may attemptto attain certain performance measures over each scheduling interval.Any performance criterion, such as maximum system throughput,proportional fairness among UE's or any other suitable criterion can beused for optimization.

In an embodiment, the eNodeB decisions are performed separately andindependently at each cluster. In various embodiments, the eNodeB's ofeach cluster communicate with one another for sharing the feedback theyreceive and for coordinating the beam setting patterns and decisions.Alternatively, the decisions are made by a centralized processor (notshown in the figures) for the entire system 20. In these embodiments,the different eNodeB's typically communicate with the centralizedprocessor for reporting the feedback they receive and for acceptingconfiguration instructions. In an example embodiment, the centralizedprocessor instructs the eNodeB's, for each beam, (1) whether the beam isto be transmitted or not, (2) the UE to which the beam is addressed(several beams can be addressed to the same UE), and (3) the modulationand coding scheme to be used in transmitting the beam.

In various embodiments, attenuation of a given beam is suitablyperformed in various ways. In some embodiments, the eNodeB's inhibittransmission of interfering beams altogether. In alternativeembodiments, the eNodeB's reduce the transmission power of the beambelow its normal intended level by a certain amount. In someembodiments, attenuation factors on the order of 10-dB are used. Inalternative embodiments, smaller attenuation factors, e.g., between 3-6dB, are suitable for enabling acceptable SNR on the preferred beams.Further alternatively, any other suitable attenuation factors can beused. In some embodiments, the eNodeB's select the actual level ofattenuation to be applied to each attenuated beam.

In some embodiments, a beam that is being attenuated is still used fortransmitting signals to other UE's. For example, a beam that isattenuated by 3-6 dB, or more, so as not to interfere with transmissionto a first UE, is often sufficiently strong to be useful for signaltransmission to a second UE. In some embodiments, the eNodeB's considerthis effect when deciding on the desired attenuation factors ofdifferent transmission beams, so as to improve the overall systemthroughput.

In some embodiments, a given UE may refrain from reporting interferingbeams to the eNodeB's. For example, in an embodiment the UE may choosenot to report any interfering beam if the interference is relativelysmall. As another example, a UE may be requested by the eNodeB's not toreport interfering beams (e.g., in a specific system configuration).

As noted above, eNodeB's 24 alternate the beam setting over time inaccordance with a certain pattern. In some embodiments, the beam settingis modified relatively slowly. In some cases, modifying the beam settingat a rate that is slower than once per several TTIs (e.g., once per tenTTIs or slower) is considered slow. In other cases, modifying the beamsetting at a rate that is slower than once per TTI is considered slow.In these embodiments, the system typically has sufficient time tocomplete a feedback cycle (i.e., measure signal quality and reportfeedback by the UE's, transmit downlink signals in response to thefeedback by the eNodeB's, and receive the downlink signals by the UE's),all while the same beam setting is still valid.

In some embodiments, the pattern of beam settings is known in advance,so that a UE measure signal quality and report feedback on a given beamsetting, which will occur again at a known future time. In other words,the UE can measure the interference inflicted by a certain beam onanother beam in a current beam setting, and predict that thisinterference will also occur in a future occurrence of this beamsetting. In an embodiment, the eNodeB's use this feedback for makingscheduling decisions and attenuating beams at the future occurrence ofthis beam setting. This technique enables the system, for example, tocompensate for the delays associated with the feedback process. Atechnique of this sort is suitably used when the beam settings changerapidly, e.g., every TTI, although it is also applicable forslowly-varying patterns.

FIG. 3 is a graph showing a transmission protocol that uses acoordinated pattern of transmission beams, in accordance with anembodiment of the present disclosure. The present example refers to twoeNodeB's, each transmitting a single beam. In the present example, thetwo eNodeB's change the beam setting every TTI. The first eNodeB appliesa periodic pattern of beam settings having a period of L₁ TTIs. Theweight vectors in this pattern are denoted w₁(1), w₁(2), . . . , w₁(L₁).The second eNodeB applies a periodic pattern of beam settings having aperiod of L₂ TTIs. The weight vectors in this pattern are denoted w₂(1),w₂(2), . . . , w2(L₂). A graph 104 in FIG. 3 shows the first pattern ofbeam settings transmitted by the first eNodeB, and a graph 108 shows thesecond pattern of beam settings transmitted by the second eNodeB.

In each TTI, each eNodeB transmits pilot signals that enable the UE's tomeasure signal quality on the two beams and calculate the feedback, asexplained above. Since the two patterns are periodic, any occurrence ofa pair of {w₁(i),w₂(j)} weight vectors will occur again at periodicintervals. Thus, in an embodiment, a given UE suitably measures signalquality and reports feedback to the eNodeB's on a certain occurrence ofa certain weight vector. The eNodeB's apply the decisions taken withrespect to this pair of weight vectors (e.g., scheduling or attenuatingbeams) in a future occurrence of this pair of weight vectors. Note,however, that the feedback typically remains valid for only a limitedlength of time, until the channel response changes considerably. Inanother embodiment, the UE can estimate the communication conditions inthe k^(th) TTI based on the signal measured at the (k-L₁)^(th) and(k-L₂)^(th) TTIs (and possibly other past measurements).

In the present embodiment, a given UE may be configured to reportvarious kinds of feedback, such as the identity of the best-performingTTI out of the next five TTIs in the pattern, the identity of the beamthat achieves the highest SNR, the achievable SNR in the best-performingTTI on the best-performing beam, and/or the achievable SNR at thebest-performing TTI on the best-performing beam assuming the other beamis attenuated. In an embodiment, based on this feedback, the twoeNodeB's cooperate to decide on the preferred scheduling policy. Thepolicy suitably involves, for example, attenuating one of the two beamsduring one or more of the TTIs. If L₁ and L₂ have no common divisor, forexample, the two eNodeB's select the scheduling policy over L₁·L₂possible beam settings.

FIG. 4 is a graph showing a transmission protocol that uses acoordinated pattern of transmission beams, in accordance with analternative embodiment of the present disclosure. In the presentexample, the system comprises three eNodeB's, referred to as eNodeB₁,eNodeB₂ and eNodeB₃. eNodeB₁ has two transmit antennas and transmits twobeams. eNodeB₂ and eNodeB₃ have one transmit antennas each, and jointlytransmit two beams. Thus, B₁=B₂={1}, B₃=B₄={2,3}, and all weight vectorsare of length 2.

In the example of FIG. 4, the eNodeB's change the beam settings (weightvectors) relatively slowly—every several TTIs. The time period duringwhich the beam setting is kept constant is denoted T_(B). In thisconfiguration, the pattern of beam settings need not be periodic or evendeterministic. For example, the pattern may be a pseudo-random pattern.

In the configuration of FIG. 4, during the first few TTIs of T_(B)(marked “DATA” in the figure) the eNodeB's transmit both data and pilotsymbols using the currently-valid beam setting. During a TTI marked“PLT”, the eNodeB's continue to transmit data using the currently-validbeam setting, but switch to transmit pilot symbols using the beamsetting of the next T_(B) period. The pilot symbols used here suitablycomprise dedicated pilot symbols. During this TTI, the UE's areconfigured to perform signal quality measurements that are applicable tothe next T_(B) period, while at the same time continuing to receive datausing the currently-valid beam setting. The UE's typically measure theachievable SNR using each of the possible beams. In alternativeembodiments, the pilots used in the “PLT” TTI suitably comprise commonpilot symbols, assuming that all weight vectors are known to the UE.

During a TTI marked “FBK”, the eNodeB's still continue to transmit datausing the currently-valid beam setting, and at the same time receive thefeedback from the UE's. This feedback that was calculated based on themeasurements performed during the “PLT” TTI, and is therefore applicableto the next T_(B) period. The eNodeB's apply this feedback (e.g., makescheduling decisions and attenuate beams) in time for the next T_(B)period.

In the example of FIG. 4, the “FBK” TTI is the last TTI in the T_(B)period, and “PLT” TTI precedes it. In alternative embodiments, the “PLT”and “FBK” TTIs are suitably positioned at any other suitable locationsin the T_(B) period, which allow sufficient time for calculating andtransmitting the feedback, and for applying it in time for the nextT_(B) period.

In some embodiments, the feedback transmitted by the UE's in the “FBK”TTI comprises, for example, the identity of the two best-performingbeams out of the four possible beams, the achievable SNR using each pairof selected beams, the best achievable SNR over the best-performing beamassuming one of the other three beams is attenuated, and/or the identityof a preferred beam to be attenuated.

After receiving this feedback from the UE's, the three eNodeB'scooperate to decide on the best scheduling scheme to be applied in thenext T_(B) period. In contrast to the example of FIG. 3 above, in whicheNodeB cooperation was needed only for attenuation purposes, in theexample of FIG. 4 eNodeB₂ and eNodeB₃ should perform the entirescheduling scheme together, since both eNodeB's transmit the same dataover their two common beams. Note also that in the present example,several TTIs may be available for scheduling during the same T_(B)period. Therefore, in the embodiment, the eNodeB's are configured tomake several scheduling decisions using the same beam setting.

In some embodiments, the T_(B) period is relatively long, so thatchanges in the communication channel during this period cannot beneglected. In these embodiments, the UE's calculate and send additionalfeedback during the T_(B) period, so as to allow better schedulingdecisions. Additionally or alternatively, prediction techniques aresuitably utilized to compensate for the feedback delay.

In some embodiments, the scheme of FIG. 4 is combined with other typesof feedback, or with other channel information available to theeNodeB's. Inasmuch as each eNodeB is free to choose its weight vectors,it can use the additional channel information to produce transmissionbeams that are more likely to be favorable by some UE's. In anembodiment, the choice of weight vectors is performed with little or nodegradation in downlink performance.

In embodiments described above, the disclosed techniques are carried outby all eNodeB's 24 of system 20 and over the entire spectrum allocatedto system 20. Generally, however, the disclosed techniques are notlimited to this sort of implementation by all eNodeB's. In alternativeembodiments, for example, the disclosed techniques are carried out usingonly part of the eNodeB's in system 20, and/or over only part of thespectrum allocated to the system.

It is thus noted that the embodiments described above are cited by wayof example, and that the present invention is not limited to what hasbeen particularly shown and described hereinabove. Rather, the scope ofthe present invention includes both combinations and sub-combinations ofthe various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method for communication, comprising: at a mobile communication terminal, receiving from a group of two or more base stations multiple Radio Frequency (RF) transmission beams that alternate in time and space and comprise at least a first beam and a second beam; estimating in the terminal first and second signal qualities for the second beam, under respective first and second constraints, wherein the first constraint specifies that the first beam is active and the second constraint specifies that the first beam is inactive; predicting in the terminal, based on the first and second signal qualities, that the first beam will interfere with reception of the second beam in a future occurrence of the first and second transmission beams; and sending from the terminal to one or more of the base stations feedback indicating that the first beam is predicted to interfere with the second beam, for use in configuring transmission to the terminal during the future occurrence.
 2. The method according to claim 1, wherein sending the feedback comprises sending to the one or more of the base stations a request to attenuate the first transmission beam during the future occurrence.
 3. The method according to claim 2, comprising transmitting data from the base stations to at least one other terminal over the attenuated first transmission beam.
 4. The method according to claim 1, wherein receiving the transmission beams comprises receiving the first and second transmission beams that alternate in time and space in accordance with a pattern that is coordinated among the base stations.
 5. The method according to claim 1, comprising identifying the second transmission beam as being preferable for receiving subsequent transmissions to the terminal, and then predicting that the first transmission beam will interfere with the identified second beam.
 6. The method according to claim 1, comprising identifying the second transmission beam as being preferable for receiving subsequent transmissions to the terminal, wherein sending the feedback comprises sending a request to receive subsequent transmissions over the second transmission beam.
 7. The method according to claim 1, wherein estimating the first and second signal qualities comprises receiving pilot signals and measuring the first and second signal qualities over the received pilot signals.
 8. The method according to claim 1, comprising selecting a preferred time interval for receiving the signals on the second transmission beam, wherein sending the feedback comprises requesting the base stations to attenuate the first transmission beam during the preferred time interval.
 9. The method according to claim 1, wherein receiving the transmission beams comprises receiving signals conforming to a Long Term Evolution (LTE) specification.
 10. The method according to claim 1, wherein receiving the transmission beams comprises receiving at least one transmission beam that is transmitted jointly by two or more of the transmitters.
 11. A communication apparatus, comprising: a receiver, which is configured to receive from a group of two or more base stations multiple Radio Frequency (RF) transmission beams that alternate in time and space and comprise at least a first beam and a second beam; and a processor, which is configured to estimate first and second signal qualities for the second beam, under respective first and second constraints, wherein the first constraint specifies that the first beam is active and the second constraint specifies that the first beam is inactive, to predict, based on the first and second signal qualities, that the first beam will interfere with reception of the second beam in a future occurrence of the first and second transmission beams, and to send to one or more of the base stations feedback indicating that the first beam is predicted to interfere with the second beam, for use in configuring transmission to the terminal during the future occurrence.
 12. The communication apparatus according to claim 11, wherein the processor is configured to send to the one or more of the base stations a request to attenuate the first transmission beam during the future occurrence.
 13. The communication apparatus according to claim 11, wherein the receiver is configured to receive the first and second transmission beams that alternate in time and space in accordance with a pattern that is coordinated among the base stations.
 14. The communication apparatus according to claim 11, wherein the processor is configured to identify the second transmission beam as being preferable for receiving subsequent transmissions to the receiver, and then to predict that the first transmission beam will interfere with the identified second beam.
 15. The communication apparatus according to claim 11, wherein the processor is configured to identify the second transmission beam as being preferable for receiving subsequent transmissions to the receiver, and to send to the one or more of the base stations a request to receive subsequent transmissions over the second transmission beam.
 16. The communication apparatus according to claim 11, wherein the receiver is configured to receiving pilot signals, and wherein the processor is configured to measure the first and second signal qualities over the received pilot signals.
 17. The communication apparatus according to claim 11, wherein the processor is configured to select a preferred time interval for receiving the signals on the second transmission beam, and, by sending the feedback, requesting the base stations to attenuate the first transmission beam during the preferred time interval.
 18. The communication apparatus according to claim 11, wherein the receiver is configured to receive at least one transmission beam that is transmitted jointly by two or more of the transmitters.
 19. A mobile communication terminal comprising the communication apparatus of claim
 11. 20. A chipset for processing signals in a mobile communication terminal, comprising the communication apparatus of claim
 11. 